This class of mediators is synthesized de novo, following mast cell activation. The two most prominent classes of mediators are arachidonic acid (AA) metabolites (eicosanoids) and cytokines.
AA is a derivative of dietary linoleic acid and is located in the membrane phospholipids of inflammatory cells. The mobilization of AA arises when mast cell activation stimulates membrane phospholipase A2 (PLA2), this being the rate limiting step in AA metabolism.
1.4.2.1 Arachidonic acid metabolites
AA metabolites consist of PGs, LTs and TXs. They result from two differing pathways, cyclo-oxygenase and lipoxygenase. The enzyme cyclo-oxygenase converts A A into the intermediate 11-hydroperoxyeicosatetraenoic acid (11-HPETE) and then sequentially into the cyclic endoperoxides PGG2 and PGH2. The PGH2 is further metabolized to form TXA2 and TXB2 and the biologically active PGD2, PGE2, PGF2 and PGI2 [107]. Different eicosanoids show marked quantitative and qualitative differences in their respective properties [107,109]. Prostaglandins of the E and F series are known to have potent actions on smooth muscle of the GI, vascular and reproductive systems [109]. PGE2 and PGF2 both contract longitudinal intestinal muscle in vitro, but while PGF2 also contracts circular muscle, PGE2 usually relaxes it. Also, PGE2 is a vasodilator but PGF2 has little effect on the vasculature. PGI2 is a vasodilator and inhibits platelet aggregation, while TXA2 is a vasoconstrictor and is proaggregatory. A property of particular interest is the inhibitory effect on gastric acid secretion which is specific to PGs of the E series and also PGI2 [110]. This effect is a regulatory mechanism which is suppressed in the presence of aspirin and other non steroidal anti-inflammatory drugs which inhibit cyclo-oxygenase. These compounds are also known to have a harmful effect on the mucosal lining of the GI tract. Administering PGs and prostacyclin leads to protective effects against these mucosal damaging agents. This raises the problem that a lack of PG production or a change
damaging and defending factors on which mucosal integrity depends.
The second enzyme, 5-lipoxygenase, results in the production of the LTs. AA is oxidized, with the activity of lipoxygenase enzymes as the rate controlling factors, to 5-hydroperoxyeicosatetraenoic acid (5-HPETE) which is reduced by glutathiol- peroxidase to the corresponding hydroxy acids (5-HETES) or the unstable peroxide LTA4 [107,108].
LTA4 has many possible routes of metabolism. It can be hydrolysed to 5(S), 12(R)- dihydroxy-6 , 14-cis-8, 10 trans eicosatetraenoic acid (LTB4) under the influence of a soluble hydrolase, whilst in a non-enzyme environment it is hydrolysed to the 5, 12- and 5, 6-dihydroxy acids. The addition of the tripeptide glutathione by a specific glutathione S-transferase produces 5(S) hydroxy-6(R)-S-glutathionyl-7,9, trans-11,14, cis-eicosatetraenoic acid (LTC4). LTC4 may be metabolized successively with the removal of glutamic acid by y-glutamyl transpeptidase to give LTD4 and the removal of glycine by cysteinyl glycinase giving LTD4. These final three leukotrienes (LTC4, LTD4 and LTE4) were first described as slow reacting substances of anaphylaxis (SRS- A) about fifty years ago [111-113].
LTs are very potent vasoactive and spasmogenic substances, and are more potent than histamine in inducing human bronchial smooth muscle constriction [115]. LTB4, LTC4 and LTD4 have profound effects on a wide variety of tissues, principally on cells of the immune system and on smooth muscle and these effects are mediated via specific membrane receptors [115]. The smooth muscle effects include the respiratory and cardiovascular systems and the GI tract [107,108,114,115].
LTB4, which induces adhesion of neutrophils to the endothelium, is also a potent chemotactic and cytokinetic compound [114,116]. In addition, LTB4, LTC4, and LTD4 appear to play roles in interferon (IFN) production [117].
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L T Q , LTD4, and LTE4 are potent smooth muscle contractors that produce prolonged contraction of smooth muscle from pulmonary, reproductive, vascular and GI tissues [107,108,115]. In addition, they induce mucus secretion and bronchoconstriction in a variety of species [115].
In the heart, LTs exert a negative inotropic effect that is species dependent, and may have a direct or indirect effect on calcium influx [118].
Many of the effects of LTs are cell specific, thus LTB4 has potent chemotactic activity for eosinophils, monocytes and neutrophils. Also, human eosinophils and neutrophils synthesize large amounts of LTB4 and LTC4 respectively. This selectivity originates from receptor specificity as LTs and also PGs are known to produce their effects through defined receptors. These receptors have been classified on the basis of second messenger systems, radioligand binding studies, purification and biophysical properties [110,119-121] [Fig 1.4].
1.4.2.2 Cytokines
The cytokines are a series of immuno-regulatory proteins of low molecular weight (< 80,000). They play numerous interacting roles in the function o f the immune system. In most cases, cytokines mediate their effects via cell surface receptors on relevant target cells and appear to act in a manner similar to that of hormones. In other cases cytokines may have anti-proliferative or anti-microbial effects upon foreign cells. The first group of cytokines discovered were the interferons, followed by the colony stimulating factors (CSFs), which regulate the proliferation and differentiation of bone marrow progenitor cells. Most of the more recently discovered cytokines have been classified as interleukins (ILs) and numbered in the order of their discovery [122].
Mast cells have been shown to release cytokines and contain cytokine messenger ribonucleic acid (mRNA) [122-124]. The stimulation of rodent mast cell lines has been shown to cause an increase in mRNA for a number of cytokines [125,126]. ILs- 3,4 and 10 have also been shown to affect mast cell growth and maturation [124,127].
and skin mast cells release TNF cx/p on immunological stimulation [128,129]. This accumulation of data is beginning to give the mast cell more prominence in the modulation of inflammatory processes in the late phase reaction, as cytokine release has been observed in association with mast cell activation during agonist induced asthma, cold-induced urticaria, or allergic rhinitis [104,130,131]. It has recently been demonstrated that mast cell lines and skin mast cells produce IL-8. In stimulated skin mast cells, immunoreactive material was positioned along cytoplasmic granules and membranes, while no preformed IL- 8 was found in unstimulated cells [132].
In conclusion, mast cell lines and freshly isolated mast cells all have the capacity to produce lymphokines and cytokines. The cytokines produced by these cells include IL-3, EL-4, IL-5, IL-6 , EL-8, granulocyte-macrophage colony stimulating factor (GM- CSF) and TN F-a, all of which could have very important roles in the regulation of inflammatory responses. It is particularly striking that these cytokines all have potential roles in immunologically important responses, most notably in orchestrating the events associated with allergic inflammation.